Freezing a slurry of particulate ceramic material to form ceramic product has been disclosed in U.S. Pat. Nos. 3,177,161; 3,512,571; 3,816,572 and 3,885,005. The production of these ceramic structures in accordance with the prior art by mixing an aqueous freeze-sensitive silica sol with the ceramic grains to form an aqueous slurry, freezing the slurry to form a frozen body, thawing and drying the body, and then firing the dried body has resulted in products which often contain large voids which adversely affect the strength of the ceramic product. The general uniformity of the products is also inconsistent and generally of poor quality which in turn adversely affects its desired properties such as thermal conductivity and thermal shock resistance. The products also frequently rupture, crack or distort during firing. Such products also tend to expand beyond the tolerances desired during the firing step or during subsequent use.
These deficiencies in the resulting ceramic products are believed to be caused by nucleation at one of the cooling surfaces due to contact with an ice crystal or with the mold surface that has a nucleation temperature higher than that of the slurry itself as the dispersion is cooled to its freezing point. This nucleation is believed to cause large ice crystals to grow out from these points and eventually entrap the last remaining liquid, resulting in rupture, cracking, weak and, non-uniform ceramic structures.
Many attempts have been made to overcome this problem such as thorough waxing of the molds, the use of different mold materials, thorough cleaning of mold surfaces, even with acids, but the problems still persisted.